1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus.
2. Description of the Related Art
There have recently been increased demands for high image quality and high stability in the electrophotography process for image forming.
Exemplary ways to improve the image quality in the electrophotography process include a method for reducing the exposure beam diameter. A smaller exposure beam diameter allows smaller electrostatic latent images to be formed, so that a higher resolving power can be achieved.
When an electrostatic latent image is formed with a smaller exposure beam diameter, however, not only the image height can be controlled less easily, but also the costs for forming images are increased.
The cost for controlling the image height of electrostatic latent images formed with a smaller exposure beam diameter comes to occupy a higher proportion of the entire costs of the image forming apparatus.
Thus, in the electrophotography process, an alternative for forming very small electrostatic latent images without reducing the exposure beam diameter has been sought for.
Furthermore, in the conventional image forming method, the height of attached toner on a line image, that is, the pile height of a line image is different from that on a solid image. Such a difference in the pile heights is caused by a difference in the sizes of the electrostatic latent images of these images.
In consideration of the demand for improved image quality and the demand for reduction in the environmental burden, the pile height needs to be controlled to an appropriate level.
The pile heights of line images and solid images may be controlled by performing some process in the developing process.
To control the pile height in the developing process, however, the latent images of a line image and a solid image need to be developed at different sensitivities because the electrostatic latent images of a line image and a solid image are different in size.
This method for controlling the pile height by developing the latent images of a line image and a solid image at different sensitivities is not preferable, because such a method causes some defects, e.g., lost faithfulness, in the resultant latent images.
In image forming, controlling the pile height without performing any process in the developing process is therefore desirable. Desired in an image forming method is a way in which an electrostatic latent image is formed in such a manner that any variation resulting from the electrophotography process, without limitation to the pile height, is compensated.
In the technology disclosed in Japanese Patent Application Laid-open No. 2005-193540, for example, when the area of an input image is smaller than a predetermined size in image forming, the exposure energy per unit pixels is increased from a level used when written is a solid image.
Another exemplary technology disclosed in Japanese Patent Application Laid-open No. 2007-190787 corrects the image data by removing some pixels from or adding some pixels to the image to be exposed so that the optical energy output from each light source becomes uniform.
In image forming, there is also a demand for output images allowing very small characters, particularly outlined character images, that is, outline characters, in a size of two or three points to be recognized at a high dot density, e.g., at 1200 dpi.
While some improvements have been made in the developing, the transfer, and the fixing processes of the image forming to allow high quality images to be output at a high dot density, such outputting of high-quality image has still been difficult.
While micron-order measurements of electrostatic latent images have been conventionally difficult, such measurements can now be conducted highly precisely. Such measurements have uncovered that a latent image, which is an image before development, was a cause of image quality deterioration in the image forming process.
In other words, uncovered by the measurements was that, when output as an image pattern was an outlined image, the electric field vector in the latent image in the vertical direction of the sample was not exactly the reverse of the electric field vector in the latent image of the ordinary image, and the vector in the outlined image was smaller than that intended in the image pattern.
In other words, when a very small image is to be output at a high dot density in the image forming, a latent image resulting from an image pattern signal supplied from the controller does not match the image pattern affected by the beam size or the electric charge diffusion. As a result, high-quality image outputs are still difficult even if any improvements in the developing, transferring, and fixing processes are made in an image forming method.
An effective way to output images of outlined characters, in particular, at high image quality is to increase the electric field vector in the latent image in the sample vertical direction toward the side not causing the toner to be attached. From the view point of electromagnetism, the simplest way to increase the electric field vectors in a white part of an image is to increase the amount of electric charge in the white part, but it is difficult to increase a local amount of electric charging.
Therefore, it is desirable to provide an image forming method capable of forming high-quality images of image patterns including image portions that include very small pixels and non-image portions.